01 - Preheat oven to 350°F. Lightly grease a 9x13-inch baking pan with non-stick cooking spray.
02 - Prepare cake batter according to package directions. Pour batter into prepared pan and bake for 25–30 minutes, or until a toothpick inserted in the center comes out clean.
03 - Remove cake from oven and let cool for 10 minutes. Using the handle of a wooden spoon, poke holes every 1–2 inches across the surface of the warm cake.
04 - In a large bowl, whisk together instant pudding mixes and cold milk for about 2 minutes, until mixture begins to thicken slightly.
05 - Divide pudding evenly among 4 separate bowls. Add a different gel food coloring to each bowl, mixing thoroughly until vibrant colors are achieved.
06 - Spoon the colored puddings randomly into the holes, creating a rainbow pattern across the cake.
07 - Spread whipped topping evenly over the entire surface of the cooled cake.
08 - Sprinkle unicorn or colorful confetti sprinkles over the top. Refrigerate for at least 1 hour before slicing and serving.